Generally speaking, the efficiency of larger speaker boxes are better than smaller boxes with similar drivers, they also can produce higher volume with less distortion. In other words, their ability to maintain a consistent and stable quality sound on both low and high volumn are better than smaller boxes. So it depends on the volumn you play, Q8 may or may not necessary. Musically speaking, they are really very close, with Q8 has little better base but with M60 as front, the difference would not be significant. If base is important to you, a subwoofer may add more value to your set up than Q8.
Another factor is phycological, I use Q8 and V150 in a 12X12 room, fully aware that they are overkill but I am happy ;-)
